This invention relates to a switching regulator power supply capable of operation from either an AC or a DC power source; and to means enabling such a power supply to effect transitions between said power sources without significant interruption.
Due to the sensitivity of modern electronic equipment to interruption of the power supplied thereto, attention has been given to the development of so-called uninterruptible power supplies capable of operating from the AC power line under normal conditions, and of immediate transition to operation from a DC power source such as a battery when the AC power line voltage drops to zero (power failure) or below its normal value (power "brownout"). Such power supplies have usually been of the variable duty cycle switching regulator type.
One such type of uninterruptible power supply is described in U.S. Pat. No. 4,038,559; and an improved type of uninterruptible power supply is described in U.S. Pat. No. 4,238,690 [the drawbacks of the circuit of U.S. Pat. No. 4,038,559 being summarized in the introductory portion of U.S. Pat. No. 4,238,690]. The present invention is an improvement on the type of uninterruptible power supply described in the latter patent; and aspects of the present invention relating to the improved stability realized by its programmed current feedback circuit are analyzed in applicant's paper published in March 1983 and entitled "A New Switched-Mode Power Conversion Topology Provides Inherently Stable Response", presented at POWERCON 10--Tenth International Solid-State Power Electronics Conference and Exhibit, San Diego, Calif.
Most modern electronic circuits, such as computers, data terminals and communication equipment, require a quite reliable DC power source. In many cases, it is required that the supply of DC power to the load be uninterrupted even when the AC power line has failed or is subject to brownout. This requires a system which includes, in addition to the primary AC power source, a reserve power source such as a battery.
Rectifiers connected to the commercial AC power line contain harmonics of the (usually 50 Hz. or 60 Hz.) AC line frequency ("ripple") in the rectifier output voltage and current. These harmonics are generally reduced in prior art designs by means of large and costly filter components.
U.S. Pat. No. 4,238,690 is representative of the present state of the art in the design of uninterruptible power supplies which seek to meet the aforementioned requirements. The uninterruptible power supply of U.S. Pat. No. 4,238,690 utilizes a highly reactive linear power transformer, rectifying diodes and a switching regulator network which closely resembles a DC to DC buck-boost type switching regulator. A semiconductor switch is connected in common to both the power transformer/rectifying diode combination and the switching regulator network. Pulse width (i.e. variable duty cycle) modulation of the semiconductor switch controls the flow of power in the system.
In the circuit of U.S. Pat. No. 4,238,690, when the commercial AC line voltage is equal to or above its normal minimum value, energy is caused to flow from the AC line to the load. When the AC power falls below said minimum value, the semiconductor switch changes duty cycle, causing the energy for the load to be drawn from a battery connected to the switching regulator network.
The circuit described in this patent, however, suffers from certain drawbacks, viz:
a. In the circuit of U.S. Pat. No. 4,238,690 the direction of energy flow is controlled by adjusting system parameters; however, this arrangement presents a difficult control design problem, since the semiconductor switch is operated off-ground. Noise generated by the system can cause the semiconductor switch to malfunction and cause a system failure.
b. R. D. Middlebrook and Slobodan Cuk, in their treatise on "Advances in Switched Mode Power Conversion", Volumes 1 and 2, Tesla Co., Pasadena, Calif., have shown that buckboost circuit topologies--a class to which the circuit of U.S. Pat. No. 4,238,690 belongs--contain zeros in the right-half plane of the control to output transfer function. Such zeros cause instability in a regulated power supply system. Thus the regulating system disclosed in U.S. Pat. No. 4,238,690 is inherently unstable and therefore unsatisfactory for its intended use.
c. In the above patent, the relationship between the power transformer input voltage and the secondary output voltage is strictly a function of the duty cycle of the semiconductor power switch, defined as the ratio of the switch "on" time to the switching period. When the secondary output voltage is low (such as 5 volts--required by many computers, data modems and other logic circuits), the current in the semiconductor switch is high, adversely affecting the reliability and operating temperature range of the switch. This high switch current could possibly be reduced by adding a secondary winding to inductor 14 of the circuit shown in the aforementioned U.S. Pat. No. 4,238,690. The addition of this winding would, however, result in another problem, since energy trapped in the primary leakage reactance of such an arrangement, would be discharged across the semiconductor switch. Usually, the magnitude of this energy exceeds that which can be safely discharged by the switch, and some auxiliary means of absorbing all or a major portion of the energy is required. Snubber networks must then be used to dissipate this energy as heat to reduce the stress on the semiconductor switch. The energy released as heat lowers the overall efficiency of the power supply.
Accordingly, an object of the present invention is to provide an improved power supply capable of utilizing both AC and DC power sources; and an uninterruptible power supply overcoming drawbacks of the aforementioned prior art power supplies.